1. Field of the Invention
This invention relates to a taking lens that is capable of preventing the deterioration of an image caused by an accidental inclination or so-called camera shake when a picture is being taken.
2. Description of the Prior Art
When taking a picture from a moving automobile or airplane, or when a long focus lens is used, a significant image deterioration, caused by a camera shake, tends to occur. Heretofore, many proposals have been made to provide means for compensating for an image blur caused by the camera shake.
In the prior art, an image blur, due to a camera shake, is compensated for, by employing a taking lens system which includes a prism. The prism is inclined with respect to the optical axis in order to deflect an optical path to an image surface.
According to the prior art, however, it is necessary that a plurality of prisms be provided in order to thoroughly compensate for an image blur caused by the camera shake or otherwise the taking lens is used in combination with a wide converter. Therefore, there arises such problems as that the control is complicated and the apparatus becomes bulky. Also, it has such an inconvenience as that a chromatic aberration is inevitable to be generated when a prism is used.
In order to solve such problems inherent in an apparatus using a prism, some proposals have been made in which an image blur is compensated by moving a part of a lens group in the vertical direction with respect to the optical axis.
Those proposals are as follows, for example.
Japanese Patent Early Laid-open Publication No. Sho 63-201623 discloses a lens system in which a lens group nearest to an image in the lens system is moved. Japanese Patent Early Laid-open Publication No. Sho 63-201624 discloses a lens system in which an extender system disposed on the side of an image of a master lens is moved. Similarly, Japanese Patent Early Laid-open Publication No. Sho 63-201622 discloses a lens system in which an afocal system is disposed on the side of an object of a master lens or on the side of an image of the master lens is moved.
In general, when a lens group among a plurality of lens groups is moved in the vertical direction with respect to the optical axis as a compensating lens, if the moved amount of the lens group is represented by Y, a moved amount .DELTA.Y of an image on an image surface caused by the movement of the lens group can be expressed by the equation: EQU .DELTA.Y=(-m.sub.A +m.sub.B).multidot.Y
wherein m.sub.A denotes a magnification composed by lens groups from the compensating lens group to the image surface, and m.sub.B denotes a magnification composed by lens groups on the side of an image to the compensating lens group.
Also, if a coefficient .alpha. regarding the compensation of movement of the image is defined in accordance with the following equations, the absolute value of the coefficient .alpha. is desirably small when the image blur is compensated by moving a part of the lens group: EQU Y=.alpha..multidot..DELTA.Y EQU .alpha.=(-m.sub.A +m.sub.B).sup.-1
If the absolute value of the coefficient .alpha. is large, the outer diameter of a lens barrel becomes large in order to obtain a space for moving the lens therein and a movable area of an actuator.
Also, in order to prevent a reduction in the intensity of a marginal ray caused by a movement of the compensating lens group, the effective diameter of the compensating lens group itself must be large. In this case, a large power actuator is required in order to move a heavy compensating lens.
When an object is located at infinity, if a focal length of the lens group on the side of the object to the compensating lens group is represented by f.sub.A and a focal length from a lens nearest to the object to the compensating lens group is represented by f.sub.B, the magnifications m.sub.A and m.sub.B can be expressed by the following equations: EQU m.sub.A =f/f.sub.A EQU m.sub.B =f/f.sub.B
Accordingly, the compensation coefficient .alpha. can be expressed as follows: ##EQU1##
Next, the above equation is applied to the afore-mentioned three Japanese Publications. As the lens system described in Japanese Patent Early Laid-open Publication No. Sho 63-201623 is constructed such that a compensating lens group is disposed behind a generally afocal lens, the following equations are obtained: EQU f/f.sub.A .apprxeq.0, f/f.sub.B .apprxeq.1
and the coefficient becomes .alpha..apprxeq.1. In a lens system of a type in which the lens group behind the afocal system is served as a compensating lens group, an equation of .alpha..apprxeq.1 can always be obtained.
In the lens system described in Japanese Patent Early Laid-open Publication No. Sho 63-201624, f/f.sub.A becomes an image magnification of the extender system itself and becomes as follows: EQU f/f.sub.B .apprxeq.1
Therefore, unless the magnification of a rear attachment lens exceeds 2, a relation of .vertline..alpha..vertline.&lt;1 cannot be satisfied. When the compensating lens group is disposed on the side nearest to the image, in order to satisfy a relation of .vertline..alpha..vertline..ltoreq.0.5, it is necessary that the focal length f.sub.A of the lens group on the side of the object than the compensating lens group is made smaller than -f or otherwise bigger than f/3.
These values are very far away from values which a usual telephotographic lens has. If the lens group on the side of the object is a strong diverging lens group, there occurs an increased total length of the lens system, eclipse of marginal ray caused by the large diameter of an exit pupil, etc. On the contrary, if the lens group on the side of the object is a strong convergent lens group, it is inevitable that the chromatic aberration and curvature of field are increased.
The lens system described in Japanese Patent Early Laid-open Publication No. Sho 63-201622 is constructed such that an attachment lens including a compensating lens group is mounted on a master lens. In this case, among lens groups comprising the attachment lens, even if the lens group on the side of the object is the compensating lens group, or even if the lens group on the side of a main lens is the compensating lens group, .alpha. becomes a value obtained by dividing the focal length of the lens group on the side of the object of the attachment lens by the focal length of the whole system. The lens system shown in this Publication includes the attachment lens on the side of the object of the master lens, and the lens group on the side of the master lens in the attachment lens is served as the compensating lens.
According to this construction, although the coefficient .alpha. becomes as small as 0.6, affection which the chromatic aberration generated in the compensating lens group renders the chromatic aberration of the whole system large. Therefore, in order to secure a sufficient performance able to be used as a taking lens, it is necessary to compensate the chromatic aberration using four pieces of lens or more. As a result, it is unavoidable that the whole lens system becomes large.